vm: Fix typo.

Regression introduced in e38e18ff.

* guix/build/vm.scm (initialize-hard-disk)[partition]: Use a string.

guix/build/vm.scm

@@ -168,7 +168,7 @@ (define target-store
     (string-append target-directory (%store-directory)))
 
   (define partition
-    (string-append device 1))
+    (string-append device "1"))
 
   (unless (initialize-partition-table device
                                       #:partition-size